Only a few hundred people braved the heat of an un-air-conditioned theater on June 21, 1913, to hear the orchestra's first concert. But around 17,000, by the orchestra's estimate, turned out Friday to hear the group celebrate its 100th birthday with a free concert at Miller Outdoor Theatre.

An hour before the concert, the grassy slope leading up from the stage at Hermann Park was already nearly full. By concert time, the crowd had spilled over to the back side of the hill.

Joseph and Yarmari Mouton of Houston only heard about the birthday concert the day before, they said, but they decided right away to join in. Obviously, a lot of others had done the same.

The orchestra's free summer concerts at the park have been an annual tradition for more than 50 years. Friday's concert, opening this year's series, attracted the kind of crowd that usually shows up only for the July 4 concert.

Even while the sun was still beating down, a breeze helped cool the concertgoers as they lounged in folding chairs or stretched out on blankets. Some of them took the opportunity to slip off their shoes and enjoy the feel of grass on their bare feet. Children played on the lawn between the theater and the reflecting pool.

A half-hour or so before he and the rest of the brasses let fly with the fanfares that open Dmitri Shostakovich's "Festive Overture," trumpeter Mark Hughes lingered outside the theater and noticed the gathering crowd. Musicians respond to that, he said.

"You look out there and see a mob, and you can't help but get excited," Hughes said.

As he was getting fired up, Taylor Long and Elizabeth Barba of League City were settling down on blankets with Perdita, the beagle. Long, who hadn't heard one of the orchestra's outdoor concerts for a couple of years, brought Barba and Perdita for their first times.

"You've got the night. You've got your wine," Long said. Coming to hear the orchestra is "just a good thing to do - especially for free."

As the moon rose, associate conductor Robert Franz stepped on stage and began the concert. After "American Idol" finalist LaKisha Jones helped lead the singing of the national anthem, Shostakovich's "Festive Overture" set the tone for the evening. As the overture's last buildup approached, the brass section of the University of Houston's Spirit of Houston Marching band suddenly appeared, decked out in blazing red uniforms, to give the big finish an extra wallop. The "Festive Overture" lived up to its name.